This invention relates to control of ocular development and, more particularly, to the treatment of the eye to control the development of myopia (commonly known as nearsightedness).
It has been estimated that about one of every four persons earth suffers from myopia. About one-half or more of these cases are axial myopia, i.e., an elongation of the eye along the visual axis.
At birth, the human eye is about two-thirds adult size and is even at that size relatively short in the axial direction. As a consequence, young children tend to be farsighted. During childhood, as the eye grows, there is a compensatory fine tuning of the optical properties of the cornea and lens to the increasing ocular length. Often the entire process is virtually perfect and no correction is needed for sharp vision at distance; the eye is emmetropic. When regulatory failure in this finely tuned process occurs, it usually goes toward a lengthened eye. As a result, distant images focus in front of the plane of the retina and axial myopia results. If, on the other hand, the regulatory failure leads to an eye whose ocular length is too short, near images focus behind the plane of the retina and the result is hyperopia (commonly known as farsightedness).
Over the years, many theories have been put forth to explain the development of myopia, e.g., inheritance, excessive near work, and environmental influences such as hours of sunshine, diet, etc. From these theories many preventative measures have been proposed including spectacles, eye exercise, eye rest, cycloplegia, and other drug therapies. The clinical literature on the subject is massive.
Based on a theory that substantial use of the eye by children for reading leads to the development of permanent nearsightedness or myopia, many remedies directed at the focussing mechanism at the front of the eye have been proposed. Largely these have been attempts either to block near focus through topical application of drugs or to remove any need for near focus through use of plus lenses that in effect perform the near focus task. Topical drugs that relax the focussing muscle of the eye, the ciliary muscle, are called cycloplegics and have been available for a century.
Some clinical studies have suggested that atropine, a long-acting cycloplegic, applied topically to the eye may retard development of myopia. Atropine treatment, however, is not practical: it causes dilation of the pupil, which results in light sensitivity, and its action to inhibit ocular focussing impairs near visual work like reading. In addition to the discomfort to the patient, there are indications that excess light can harm the retina and questions have been raised concerning the danger of the long-term use of atropine (or other strong cycloplegics) on the retina when exposed to bright light.
There is now substantial evidence to link the posterior part of the eye, specifically image quality at the retina and hence an extension of the nervous system, to the postnatal regulation of ocular growth. There is significant evidence of myopia resulting in an eye that is subjected to retinal image degradation. It has been shown that axial myopia can be experimentally induced, in either birds or primates, in an eye in which the retina is deprived of formed images, e.g., by suturing the eyelids or wearing an image-diffusing goggle. The experimental myopia induced in primates such as monkeys precisely mimics the common axial myopia of humans.
Thus, the phenomenon of an animal's vision process apparently contributes to the feedback mechanism by which postnatal ocular growth is normally regulated and refractive error is determined. This indicates that this mechanism is neural and likely originates in the retina.
In the application of R. A. Stone, A. M. Laties and P. M. Iuvone, U.S. application Ser. No. 342,942, filed Apr. 25, 1989, which is a continuation-in-part of Ser. No. 202,220, filed Jun. 3, 1988, a method of controlling the abnormal postnatal growth of the eye of a maturing animal was found which comprises controlling the presence of a neurochemical, its agonist or antagonist, which neurochemical is found to be changed under conditions during maturation leading to abnormal axial length. Therein it is disclosed that in experimental animals, such as chicks or monkeys, subjected to ocular image deprivation ordinarily leading to the development of myopia, the metabolism of certain retinal neurochemicals is altered leading to changes in retinal concentrations thereof. Specifically, retinal concentrations of dopamine were found to be reduced during such image deprivation and the ocular administration of a dopamine-related agent, e.g., apomorphine, a dopamine agonist, was found to inhibit or actually prevent the axial enlargement of the eye under conditions ordinarily leading to such enlargement.
There have been recent advances made in the understanding of the cholinergic nervous system. Cholinergic receptors are proteins embedded in the wall of a cell that respond to the chemical acetylcholine. They are broadly broken down into nicotinic and muscarinic receptors. In this respect, it is now known that the muscarinic receptors are not all of one type. Recent findings show that there are at least five, if not more, types of cholinergic muscarinic receptors (types M.sub.1 through M.sub.5). Type M.sub.1 receptors are those present in abundance and thought to be enriched in the brain neural tissue and neural ganglia. Other receptors are concentrated in other tissues, such as in heart, smooth muscle tissue, or glands. While many pharmacological agents interacting with muscarinic receptors influence several types, some are known to have a major effect on a single type of receptor with relative selectivity and other agents can have a relatively selective effect on a different single receptor. Still other agents may have a significant effect on more than one or even all types of receptors. A pharmacological antagonist, for the purposes of this discussion, is an agent that effectively blocks the receptor. It is known that pirenzepine, (Gastrozepin, LS 519) 5,11-Dihydro-11-[4-methyl-1-piperazinyl)acetyl]-6H-pyrido[2,3-b][1,4]benzo diazepin-6-one, and its dihydrochloride, are known as anticholinergic, selective M.sub.1 antagonists. It is further known that telenzepine, i.e., 4,9-dihydro-3-methyl-4[(4-methyl-(1)piperazine)acetyl]1OH-thieno-[3,4 -b][1,5]-benzodiazepin-10-on, and its dihydrochloride, are also known as anticholinergic selective M.sub.1 antagonists reported to be about ten times as potent as pirenzepine. (See Euro. Jour. of Pharmacology, 165 (1989) 87-96.) It is also known that 4-DAMP (4-diphenylacetoxy-N-methylpiperadine methiodide) is a selective antagonist for smooth muscle (ordinarily called M.sub.3 type but variously called type M.sub.2 or M.sub.3, as the current classification of receptors is in flux). It is believed that atropine is an antagonist for all types of cholinergic muscarinic receptors.